Preserving a Landmark Tree at Deering Estate in Miami
The beneﬁts of trees in our parks and open spaces are well-known. Large trees provide shade, oxygen, carbon sequestering, ﬁltering of pollutants and cooling effects to name a few. They also can provide a landmark, a historical reference and a sense of well-being to those living and working around large trees.
But trees do age, like people, and develop structural defects, decay, damaged parts, poor circulation, and root problems. Eventually the defects can make a large, veteran tree unsafe or give the appearance that the tree is unsafe. And as arborists, we cannot guarantee that a tree will not fail. Every tree has some risk. So as arborists, the best we can do is evaluate the known, observable weak points and determine if the defect can be mitigated and the treeʼs health improved or sustained.On the grounds of the 444-acre Deering Estate, a large black olive tree (Bucida buceras) grows at the northeast corner of the Stone House which was built in 1922 by Charles Deering, an avid art collector and chairman of the board of the newly formed International Harvester Company in Illinois from 1902 to 1910. The black olive tree does not appear on photos prior to 1930 so park officials estimate the age of the tree around 70 to 75 years old. In 1997 the tree was named a National Champion2 Black Olive by the American Forests because it is the largest black olive in the country. The tree was severely damaged in Hurricane Andrew in 1990. Although the surrounding buildings have been painstakingly restored so the effects of Andrew are no longer visible, the damage caused to the black olive has left large defects that have become part of the tree’s structure.
Arborists have some new tools to assist in the evaluation of a tree such as the Deering Estate black olive. The Resistograph is a portable drill with a graphing scale that indicates the resistance of the wood as the narrow drill bit passes through the different layers of wood. It provides a graphical picture of sound wood and decay that might be present inside the trunk and out of sight. A low-tech tool, the rubber mallet, is a simple device that can be used to locate internal decay in tree trunks and large branches. When sounding with a mallet indicates a hollow area, I followup with a Resistograph test to determine the size of the cavity and the location of the cavity or decay.
Air excavation tools, sold as either an Air Spade or an Air Knife, are other devices that allow an arborist to locate and examine roots without damaging the roots. The tool uses a large compressor which forces air at supersonic speed through a nozzle. The air blows granular material like soil away but leaves solid objects like roots in place. Roots can be uncovered, examined for decay and then recovered. The Air Spade is useful for ﬂufﬁng soil that has become compacted over time. Roots need air for respiration. The Air Spade is an excellent tool for aerating soil and reducing compaction that inhibits root growth.
The black olive tree is a popular site for weddings and other activities at the park so risk is an important consideration when evaluating the health and structure of the tree. What at ﬁrst appeared to me to be severe lightning damage to the east side of the black olive trunk turned out to be damage to the trunk caused by collision with debris during the ﬂooding and wave action of the hurricane. The bark and cambium layer on the east side of the trunk appeared to have been pealed off during the storm and over time the internal dead heartwood and sapwood have become exposed. Foot trafﬁc and regular passing of a lawn mower under the dripline of the tree also contribute to soil compaction which impedes root growth.
We drilled the trunk and root ﬂare with the Resistograph in various locations to check the soundness of the exposed heartwood and the thickness of the new solid sapwood. In spite of the exposed condition of the trunk, the exposed wood was very hard and solid and there was no apparent crack or weakened area between the sapwood and the heartwood. All Resistograph measurements including those taken on three of the root ﬂares indicated the wood was very hard and solid.
We then used an Air Spade to ﬂuff up and aerate the soil. The Air Spade was inserted vertically into the soil to a depth of about 18 inches. These vertical 2-inch diameter holes were made randomly but at an approximate distance of 2 to 3 feet apart within the dripline of the tree. Later these vertical holes were ﬁlled with composted cow manure. Also during the vertical mulching we used the Air Spade to stir and ﬂuff the top 3 to 6 inches of the surface soil. Compost was then spread over the surface of the soil. Finally, a 2 to 3-inch layer of composted organic mulch is to be spread under the tree within the dripline. This mulch layer will slowly become incorporated into the soil improving the level of organic matter, improving rooting conditions and increasing water-retention capacity of the soil. Another beneﬁt of the organic mulch is the cushioning effect which prevents soil compaction. Organic mulch also eliminates turf which competes with the tree roots for water and nutrients. Reducing competing plants around veteran trees is always helpful for the tree.
Finally, we recommend some branch end-weight reduction pruning to keep the
branches from sprawling to far out from the trunk reducing stress on branch connections at the trunk and on decayed spots in some of the branches. Many of the sprouts on the branches should be left in place and managed so new internal branches can be created helping to keep the overall spread of the treeʼs crown more compact.
Reduction pruning to a veteran tree like the Deering Estate black olive may in time cause the tree to lose its National Champion designation by gradually reducing its size. However, size reduction will make the tree safer and help prolong the treeʼs life as a signiﬁcant landmark in the park. Fertilizer is usually not recommended for treatment of veteran trees unless a speciﬁc deﬁciency symptom is noted. No nutrient deﬁciency symptoms were observed. Sometimes a tree growth regulator (paclobutrazol) is used to help promote root growth by diverting tree resources into root growth instead of top growth.
I would like to thank my colleagues Jorge Rivera, an ISA Certiﬁed Arborist with
Everglades Environmental Care, Inc., Henry Mayer, Commercial Horticulture Agent and ISA Certiﬁed Arborist with the Miami-Dade/University of Florida Extension Service, and Dr. Alice Warren, Natural Resources Manager with the Miami-Dade Park & Recreation Department for their assistance on this project.
- The American Forests Register of Big Trees designates the largest of 826 species of trees in the US. American Forests uses the following calculation to determine a tree’s total points: Trunk Circumference + Height + ¼ Average Crown Spread = Total Points. American Forests is a non-proﬁt conservation organization that was founded in 1875. (americanforests.org)